1. Field of Invention
The invention relates to a cutter for a tunnel boring machine as indicated in the introduction to claim 1.
2. Description of Prior Art
Machines for mechanical boring of tunnels are called tunnel boring machines (TBM). They have a rotating drill head, on which are mounted excavation tools for breaking off the material that has to be removed in the tunnel front. The most common excavation tools are called cutters, which break off the material that has to be removed by being pressed in against the tunnel front and forced to rotate by means of rotation of the drill head.
A cutter has a shaft, which is fixed to the drill head, and a cutter body with a steel ring—called a cutter ring—rotates about this shaft. The cutter ring, which is in direct contact with the material in the tunnel front, becomes worn and has to be replaced. As a rule the consumption of cutter rings is many times that of the cutter's other components.
Simplifying the task of replacing the cutter rings may provide substantial cost reductions.
There are cutters for TBM with undivided as well as divided cutter rings.
In principle, different techniques are employed for fixing undivided and divided cutter rings to the cutter body. Undivided cutter rings are heated and shrunk on to the cutter body, and a locking ring with a split is inserted in a groove on the side of the cutter ring, and the split is welded. The lateral surfaces in the foot of undivided cutter rings are parallel to the plane through the cutter's cutting edge. For divided cutter rings that have slanting lateral surfaces in the foot, fixing elements are used that are pressed into a wedge-shaped opening between the cutter body and the cutter ring by means of screws, thereby fixing the cutter rings, cf. U.S. Pat. Nos. 5,341,889, 4,793,427, NO patent 109859, etc. FIG. I in the attached drawings illustrates the methods of construction.
When the cutter ring on a cutter with an undivided ring has to be replaced, the whole cutter has to be dismantled and taken to the workshop in order to have a new ring mounted.
When the cutter ring on a cutter with a divided ring has to be changed, only the cutter ring segments are replaced while the cutter remains on the drill head.
Since cutters weigh 100–200 kg, while the cutter ring weighs 20–30 kg, a great deal of time and work is saved with regard to ring replacement with divided cutter rings, as well as reducing the risk involved in the operation.
There is substantial cutter consumption in the case of hard and abrasive rock. The potential savings that can be made by using divided cutter rings are greatest in such conditions.
Experience with divided cutter rings has shown a tendency for fracture to occur in the gap between the ring segments in some cutter positions, out towards the periphery of the TBM drill head. Here the cutter rings are exposed to considerable lateral loads due to the fact that the cutters are at an angle to the TBM axis. These positions normally have a very high replacement frequency in hard rock.
A fair amount of variation is usually found in the rock formations in the tunnel route. This makes it desirable to have the ability to alternate between cutters with divided and undivided rings as the working conditions change. Since the known constructions of cutters with whole and divided rings are so different, with the current technology it is not possible to alternate between whole and divided cutter rings without replacing the entire cutter system. This will entail an increased number of spare cutters and reduce the savings achieved by using divided rings. For this reason cutters with divided rings have had little impact on the market, despite the substantial savings potential.
In Norwegian patent NO 169859 (belonging to the applicant) a clamping ring is employed for securing a cutter ring consisting of several segments. The clamping ring is designed with a slanting lateral edge that is adapted to the opposite slanting lateral surface of the cutter ring (complementarily shaped), and the clamping ring is pulled into position in a suitable groove in a cutter ring, thereby holding the cutter ring in place by means of its wedge shape.
Furthermore, in U.S. Pat. No. 4,793,427 a ring is described for securing a cutter ring (13) in a tool. The ring is pulled in a threaded connection between the ring and the cutter, thereby holding the cutter in place by means of two annular shoulders without exerting radially outwardly directed forces on the cutter ring. It should also be noted that the cutter ring in U.S. Pat. No. 4,793,427 is a hard metal ring with a low interference fit (sliding fit) to the cutter body. A hard metal ring as indicated in U.S. Pat. No. 4,793,427 is used in shaft boring and not in a TBM. Rings of this kind cannot be used with the cutter pressures of 20–30 tons employed in TBM boring today, since they crack at such pressure.